Thermocouples

Your conception is not so clear with the thermocouple i think so. We cant calibrate a thermocouple , only we can verify it with specified range . For that you require a temperature bath and put your hot junction and apply the temp u need say 0% to 100% and measure millivolt with the help of process multi-miter. then it will sink millivolt data relates to temperature.

Now cold junction compensation

when the cold junction is at 0°C, also known as an ice-bath reference. If T_{C(cold junction)} = 0°C, then V_{OUT(millivolt)} = V_{H(Hot junction)}. In this case, the voltage measured at the hot junction is a direct translation of the actual temperature at that junction. The National Bureau of Standards (NBS) provides lookup tables containing the characterization data of thermocouple voltages vs. temperatures for various types of thermocouples. The data are all based on a cold-junction temperature of 0°C. With an ice-bath reference, you can determine the temperature of the hot junction by looking for V_H in the appropriate table.

I hope it is clear now

Sudip

Inst eng

Thermocouple can't be calibrated, however the device you are going to connect thermocouple to, can be.

As far as refrence junction is concerned, You should search for 'Thermocouple Cold Junction Compensation' either on the internet or in books.

Actually, in control rooms there is a constant temperature maintained, like 25 Degree C. So is sensed by the cold junction. The emf produced due to cold junction temperature are subtracted off the voltage produced due to Hot junction.

There more than one ways to apply cold junction compensation.

This is what I did using thermocouples. The manufacturer has a reference chart V to Temperature just follow there spec. To calibrate use known physics constants boiling water and ice water. Hope this helps.

In Industrial environment the terminology "calibration of thermocouple and Pyrometer" is a common usage. Here the calibration of a thermocouple means checking the millivolt output of the thermocouple in comparison to a standard thermocouple of the same type that is in use in the same furnace under same conditions.This is done to ensure that the thermocouple in use is giving correct output. ( The standard thermocouple is one that is checked and verified in laboratory calibrating furnace as per the chart and test schedule.)

Similarly the millivolt is injected with the help of a thermocouple test set in to the pyrometer and compare the reading with reference to the chart and ensure that meter is indicating the correct temperature in response to the thermocouple output and operate the controlling system and switching on and off the heaters at the set temperature to ensure that the process temperature inside the furnace is correct.

The room temperature correction is made after the calibration .

This is being done as a routine practice to ensure that the charge in the furnace does not get rejected due to wrong temperature in side the furnace due to the ageing of the thermocouple joint and technical error of the pyrometer which happens often. In ISO certified factories it is advised to maintain such routine calibration reports of both thermocouple and Pyrometer as a document for quality management system and verified by the auditors. There are agencies who undertake this job.

Hello,

If you are still looking for a thermocouple reference, you should visit Madison Company's website. They have a lot of easy to understand charts and other information that will help you.

goodluck!